In the tropics, the proportion of heavier water isotopes in precipitation is anticorrelated with the precipitation amount. The physical processes underlying this so‐called amount effect are still ...poorly understood and quantified. In the present study, stable water isotopes (H218O and HDO) have been introduced in a single column model including the Emanuel convection parameterization. We investigate the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions. We focus on convective processes, since the idealized framework of the single column models does not allow us to consider the effects of large‐scale horizontal advections of air masses of different isotopic signatures. We show that two kinds of processes predominantly explain the amount effect: first, the reevaporation of the falling rain and the diffusive exchanges with the surrounding vapor; and second, the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. This highlights the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of precipitation in the tropics. The variability of the isotopic composition on different timescales (from days to months) is also studied using a unidimensional simulation of the Tropical Ocean–Global Atmosphere–Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) campaign. The amount effect is best observable at intraseasonal or longer timescales. The period of time over which convective activity significantly affects the isotopic composition of precipitation is related to the residence time of water within atmospheric reservoirs.
Recent rapid Arctic sea-ice reduction has been well documented in observations, reconstructions and model simulations. However, the rate of sea ice loss is highly variable in both time and space. The ...western Arctic has seen the fastest sea-ice decline, with substantial interannual and decadal variability, but the underlying mechanism remains unclear. Here we demonstrate, through both observations and model simulations, that the Pacific North American (PNA) pattern is an important driver of western Arctic sea-ice variability, accounting for more than 25% of the interannual variance. Our results suggest that the recent persistent positive PNA pattern has led to increased heat and moisture fluxes from local processes and from advection of North Pacific airmasses into the western Arctic. These changes have increased lower-tropospheric temperature, humidity and downwelling longwave radiation in the western Arctic, accelerating sea-ice decline. Our results indicate that the PNA pattern is important for projections of Arctic climate changes, and that greenhouse warming and the resultant persistent positive PNA trend is likely to increase Arctic sea-ice loss.
The measurement and simulation of water vapor isotopic composition has matured rapidly over the last decade, with long‐term data sets and comprehensive modeling capabilities now available. Theories ...for water vapor isotopic composition have been developed by extending the theories that have been used for the isotopic composition of precipitation to include a more nuanced understanding of evaporation, large‐scale mixing, deep convection, and kinetic fractionation. The technologies for in situ and remote sensing measurements of water vapor isotopic composition have developed especially rapidly over the last decade, with discrete water vapor sampling methods, based on mass spectroscopy, giving way to laser spectroscopic methods and satellite‐ and ground‐based infrared absorption techniques. The simulation of water vapor isotopic composition has evolved from General Circulation Model (GCM) methods for simulating precipitation isotopic composition to sophisticated isotope‐enabled microphysics schemes using higher‐order moments for water and ice size distributions. The incorporation of isotopes into GCMs has enabled more detailed diagnostics of the water cycle and has led to improvements in its simulation. The combination of improved measurement and modeling of water vapor isotopic composition opens the door to new advances in our understanding of the atmospheric water cycle, in processes ranging from the marine boundary layer, through deep convection and tropospheric mixing, and into the water cycle of the stratosphere. Finally, studies of the processes governing modern water vapor isotopic composition provide an improved framework for the interpretation of paleoclimate proxy records of the hydrological cycle.
Key Points
Measurement and simulation of water vapor isotopes is now a mature field
Analysis of water vapor isotopes can provide new constraints on the atmospheric water cycle
Improved understanding of water vapor isotopes can improve interpretation of paleoclimate proxies
Cumulus convection constitutes a key process in the control of tropical precipitation and the vertical transport of atmospheric water. To better understand the influence of convective processes on ...the isotopic composition of precipitation and water vapor, water stable isotopes (H218O and HDO) are introduced into a single column model including the Emanuel convective parameterization. This paper analyzes unidimensional simulations of the tropical atmosphere in a state of radiative‐convective equilibrium, and simulations forced by data from the Tropical Ocean–Global Atmosphere–Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE). This study shows that deep convective atmospheres are associated with robust isotopic features such as an isotopic composition of the air below the tropical tropopause layer (around 12–13 km) close to the typical values observed in the lower tropical stratosphere, and an isotopic enrichment of the upper tropospheric water that starts well below the tropopause. It highlights the critical role of condensate lofting and convective detrainment in these features, and the role of convective unsaturated downdrafts in the control of the isotopic composition of precipitation. Finally, it shows that the so‐called “amount effect” primarily reveals the influence of large‐scale atmospheric circulation changes on the isotopic composition of the precipitation, and that temperature changes not associated with circulation changes lead to an “anti–amount effect”. The detailed analysis of the physical processes underlying the “amount effect” is presented in a companion paper.
We present simulations of water‐stable isotopes from the LMDZ general circulation model (the LMDZ‐iso GCM) and evaluate them at different time scales (synoptic to interannual). LMDZ‐iso reproduces ...reasonably well the spatial and seasonal variations of both δ18O and deuterium excess. When nudged with reanalyses, LMDZ‐iso is able to capture the synoptic variability of isotopes in winter at a midlatitude station, and the interannual variability in mid and high latitudes is strongly improved. The degree of equilibration between the vapor and the precipitation is strongly sensitive to kinetic effects during rain reevaporation, calling for more synchronous vapor and precipitation measurements. We then evaluate the simulations of two past climates: Last Glacial Maximum (21 ka) and Mid‐Holocene (6 ka). A particularity of LMDZ‐iso compared to other isotopic GCMs is that it simulates a lower d excess during the LGM over most high‐latitude regions, consistent with observations. Finally, we use LMDZ‐iso to explore the relationship between precipitation and δ18O in the tropics, and we discuss its paleoclimatic implications. We show that the imprint of uniform temperature changes on tropical δ18O is weak. Large regional changes in δ18O can, however, be associated with dynamical changes of precipitation. Using LMDZ as a test bed for reconstructing past precipitation changes through local δ18O records, we show that past tropical precipitation changes can be well reconstructed qualitatively but not quantitatively. Over continents, nonlocal effects make the local reconstruction even less accurate.
We provide the first continuous measurements of isotopic composition (δD and δ18O) of water vapor over the subtropical Eastern North Atlantic Ocean from mid‐August to mid‐September 2012. The ship was ...located mostly around 26°N, 35°W where evaporation exceeded by far precipitation and water vapor at 20 m largely originated from surface evaporation. The only large deviations from that occurred during a 2 day period in the vicinity of a weak low‐pressure system. The continuous measurements were used to investigate deuterium excess (d‐excess) relation to evaporation. During 25 days d‐excess was negatively correlated with relative humidity (r2 = 0.89). Moreover, d‐excess estimated in an evaporative model with a closure assumption reproduced most of the observed variability. From these observations, the d‐excess parameter seems to be a good indicator of evaporative conditions. We also conclude that in this region, d‐excess into the marine boundary layer is less affected by mixing with the free troposphere than the isotopic composition. From our data, the transition from smooth to rough regime at the ocean surface is associated with a d‐excess decrease of 5‰, which suggests the importance of the ocean surface roughness in controlling d‐excess in this region.
Key Points
Deuterium excess in low‐level water vapor is mainly controlled by humidity
Deuterium excess presents no clear influence of mixing processes
Molecular diffusivities from Merlivat 1978a are in agreement with the data
Atmospheric humidity and soil moisture in the Amazon forest are tightly coupled to the region's water balance, or the difference between two moisture fluxes, evapotranspiration minus precipitation ...(ET-P). However, large and poorly characterized uncertainties in both fluxes, and in their difference, make it challenging to evaluate spatiotemporal variations of water balance and its dependence on ET or P. Here, we show that satellite observations of the HDO/H
O ratio of water vapor are sensitive to spatiotemporal variations of ET-P over the Amazon. When calibrated by basin-scale and mass-balance estimates of ET-P derived from terrestrial water storage and river discharge measurements, the isotopic data demonstrate that rainfall controls wet Amazon water balance variability, but ET becomes important in regulating water balance and its variability in the dry Amazon. Changes in the drivers of ET, such as above ground biomass, could therefore have a larger impact on soil moisture and humidity in the dry (southern and eastern) Amazon relative to the wet Amazon.
Climate models suggest an important role for land‐atmosphere feedbacks on climate, but exhibit a large dispersion in the simulation of this role. We focus here on the role of continental recycling in ...the intraseasonal variability of continental moisture, and we explore the possibility of using water isotopic measurements to observationally constrain this role. Based on water tagging, we design a diagnostic, named D1, to estimate the role of continental recycling on the intraseasonal variability of continental moisture simulated by the general circulation model LMDZ. In coastal regions, the intraseasonal variability of continental moisture is mainly driven by the variability in oceanic moisture convergence. More inland, the role of continental recycling becomes important. The simulation of this role is sensitive to model parameters modulating evapotranspiration. Then we show that δD in the low‐level water vapor is a good tracer for continental recycling, due to the enriched signature of transpiration. Over tropical land regions, the intraseasonal relationship between δD and precipitable water, named D1_iso, is a good observational proxy for D1. We test the possibility of using D1_iso for model evaluation using two satellite data sets: GOSAT and TES. LMDZ captures well the spatial patterns of D1_iso, but underestimates its values. However, a more accurate description of how atmospheric processes affect the isotopic composition of water vapor is necessary before concluding with certitude that LMDZ underestimates the role of continental recycling.
Key Points
estimation of role of continental recycling
water isotopes reflect continental recycling over tropical land
comparison with GOSAT and TES
Extreme Precipitation in Tropical Squall Lines Abramian, Sophie; Muller, Caroline; Risi, Camille
Journal of advances in modeling earth systems,
10/2023, Volume:
15, Issue:
10
Journal Article
Peer reviewed
Open access
Abstract
Squall lines are substantially influenced by the interaction of low‐level shear with cold pools associated with convective downdrafts. Beyond an optimal shear amplitude, squall lines tend to ...orient themselves at an angle with respect to the low‐level shear. While the mechanisms behind squall line orientation seem to be increasingly well understood, uncertainties remain on the implications of this orientation. Roca and Fiolleau (2020,
https://doi.org/10.1038/s43247-020-00015-4
) show that long lived mesoscale convective systems, including squall lines, are disproportionately involved in rainfall extremes in the tropics. This article investigates the influence of the interaction between low‐level shear and squall line outflow on squall line generated precipitation extrema in the tropics. Using a cloud resolving model, simulated squall lines in radiative convective equilibrium amid a shear‐dominated regime (super optimal), a balanced regime (optimal), and an outflow dominated regime (suboptimal). Our results show that precipitation extremes in squall lines are 40% more intense in the case of optimal shear and remain 30% superior in the superoptimal regime relative to a disorganized case. With a theoretical scaling of precipitation extremes (C. Muller & Takayabu, 2020,
https://doi.org/10.1088/1748-9326/ab7130
), we show that the condensation rates control the amplification of precipitation extremes in tropical squall lines, mainly due to its change in vertical mass flux (dynamic component). The reduction of dilution by entrainment explains half of this change, consistent with Mulholland et al. (2021,
https://doi.org/10.1175/jas-d-20-0299.1
). The other half is explained by increased cloud‐base velocity intensity in optimal and superoptimal squall lines.
Plain Language Summary
Squall lines are bands of clouds and thunderstorms spanning hundreds of kilometers, also called quasi‐linear mesoscale convective systems. These systems are associated with extreme weather conditions, including extreme rainfall rates. To better understand and therefore predict this high impact phenomenon, this study investigates the physical processes leading to enhanced precipitation rates when clouds are organized into squall lines, using idealized high‐resolution simulations. Interestingly, the dynamics of squall lines, notably their wind structures, are found to play a key role in setting the intensity of extreme rainfall rates.
Key Points
Precipitation extremes are enhanced by about 30%–40% in optimal and superoptimal squall lines compared to random convection
The enhancement of extremes is due to reduced dilution by entrainment and enhanced initial vertical velocity of updrafts in optimal and superoptimal regimes
The enhanced vertical velocity in convective updrafts does not depend on the orientation of squall lines in the superoptimal regime
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